1. Field of the Invention
The present invention relates to a data communication apparatus for communicating data.
2. Description of the Prior Art
In a prior art apparatus of this type such as a facsimile machine, transmission in a high speed mode is not possible because of line conditions at the destination station. Since the transmission is, in many cases, attained in a low speed mode under such circumstances, the transmission in the low speed mode is carried out by an automatic fall-back function. In the transmission by the automatic fall-back, however, the protocol time is long and the fall-back is not certain.
If the destination station has no facsimile machine, it is not possible to register a telephone number of the destination station to a telephone number registration means at the destination station and communicate by calling by a shortened dialing number or a one-touch key of the facsimile machine.
When the facsimile apparatus has the telephone number registration means for the destination stations to automatically call the destination telephone numbers and broadcast a message to a plurality of destination stations, the transmission in only one mode is permitted.
In the past, the facsimile communication protocol followed the "Protocol for Document Facsimile Transmission in the General Switched Telephone Network" of the CCITT Recommendation T.30.
The protocols in multi-page transmission/reception in accordance with the Recommendation T.30 are explained for a past-protocol of the last document and a protocol in a transmission speed fall-back with and without mode change.
FIG. 4A shows a facsimile communication protocol without mode change. In FIG. 4A, after a sending station detects a confirmation for reception ready (CFR) signal 11a, it sends a training signal (12a) and then sends a first page of an image signal (13a). After the transmission of the image signal, it sends an end of image (RTC) signal (15a). The RTC signal comprises six continuous EOL's (each of which comprises 11 "0"'s and one "1", 000 . . . 001) and informs to the receiving station of the end of the image signal. The RTC signal is transmitted at the same transmission speed as the image signal. On the other hand, the receiving station detects the RTC signal following to the image signal (16a) to detect the end of the image signal and waits for protocol signals. If a next document is to be transmitted without the mode change, the sending station sends a multi-page (MPS) signal (17a) after the transmission of the RTC signal to inform the receiving station that it will send the image signal of the next document in the same mode as the previous image signal. The receiving station detects the MPS signal (18a) and when it detects reception ready status for the next document, it sends a message acknowledged (MCF) signal (19a) to inform to the sending station that the previous document image signal has been received, and waits for the next image signal. The sending station detects the MCF (20a), and when it is ready to send the next document, it sends a training signal (21a) and starts to send the second image signal (22a) as was done for the first image signal. The above operation is repeated so long as the document to be transmitted exists. The protocol before the transmission of the first image signal is not explained here because it has no direct relation to the present invention.
FIG. 4B shows a communication protocol when a mode change is included. In FIG. 4B, the protocol up to the detection (11b) of the CFR signal from the receiving station to the sending station and the transmission of the first image signal is similar to the protocol up to 11a in FIG. 4A. After the transmission of the first image signal, the RTC signal is sent (15b). If the mode change is required, an end of message (EOM) signal is sent (17b) to inform the receiving station to receive the second document in a different mode than the mode in which the first document was received. The receiving station sends the MCF signal (19b) after it detects the EOM signal (18b). In accordance with the Recommendation T.30, the receiving station sends a DIS signal six seconds after it sends the MCF signal to tell the function of the receiving station to the sending station (21b). The sending station detects the DIS signal (22b) and sends a DCS signal (23b) to tell the mode for the second document to the receiving station. The receiving station detects the DCS signal (24b) to recognize the mode for the second document, checks the training of the modem by the following training (TCF) signal, and if the training is correct, sends the CFR signal (27B) and receives the image signal. The sending station detects the image signal reception ready status of the receiving station by the CFR signal (28b ) and sends the image signal (30b).
FIG. 4C shows a communication protocol after the last document. After the sending station sends the image signal of the last document (1c), it sends the RTC signal (3c) and an end of protocol (EOP) signal (5c) to tell to the receiving station the last document. The receiving station detects the EOP signal (6c) to detect that it is the last document, and sends the MCF signal (7c) and waits for the DCN signal. After the sending station detects the MCF signal (8c), it sends a digital command (DCN) signal (9c) to switch the line from the facsimile machine to the telephone set. Thus, one communication cycle terminates (11c). Similarly, the receiving station detects the DCN signal (10c) and switches the line to the telephone set. Thus, one communication cycle terminates (12c).
FIG. 4D shows a communication protocol of a transmission speed fall-back. After the first image signal is sent, the sending station sends the RTC signal and an MPS/EOM signal (15d). The receiving station checks an image quality (7d), and if the image quality is poor, it sends a rejection of training (RTN) signal instead of the MCF signal to tell to the sending station that the current document image was not correctly received (8d). The sending station detects the RTN and the DCS signal (11d) again to decrease the transmission speed of the next document image signal from that of the previous document. It also sends a training TCF signal (13d) to tell to the receiving station that the transmission speed has been changed and to check the modem training in the receiving station. If the check in the receiving station is acceptable, the receiving station sends the CFR signal and receives the next document image signal (15d).
The facsimile communication system in accordance with the T.30 Recommendation requires lengthy protocol time. Accordingly, communication cost is high when a number of documents are to be transmitted or when the transmission is to be done to a distant place.